The office of President Yoon Suk Yeol said Sunday it is not letting its guard down even though the window for North Korea's satellite launch has expired, as the country can go ahead with a launch at any time.

North Korea had set a period between the start of May 31 and the start of June 11 as the window for a satellite-carrying space rocket launch. The country fired the rocket on the first day of the window, but the launch ended in failure with the rocket crashing in the Yellow Sea.

At the time, the North acknowledged the launch failed due to an engine problem and said it would try again as soon as possible. South Korean officials warned that a second launch could still come within the launch window.

"Though the notice period is over, North Korea can launch a long-range ballistic missile at any time without prior notice," a senior presidential official told Yonhap News Agency.

North Korea claims it has the right to launch a space vehicle to put a satellite into orbit, but many in the international community believe it is a disguise for a test of its intercontinental ballistic missile technology.

Under UN resolutions, the North is banned from any activity using ballistic missile technology.

"South Korea and the United States are continuing surveillance activity," the senior official said. "We will go ahead with the sharing of missile warning information between South Korea, the US and Japan as planned."

Another official also said nothing has changed even though the launch window has passed.

"We will make sure to keep our readiness posture," the official said. (Yonhap)